Frequency responsive device



Sept. 13, 1966 c ROSE 3,273,083

FREQUENCY RESPONS IVE DEVI CE Filed April 14, 1964 I NV ENTOR.

Gus C Rose BY M MaL M United States Patent 3,273,083 FREQUENCY RESPONSIVE DEVICE Gus C. Rose, Wood Dale, I11., assignor to Motorola, Inc., Franklin Park, 111., a corporation of Illinois Filed Apr. 14, 1964, Ser. No. 359,582 3 Claims. (Cl. 33373) This invention relates generally to wave signal selection, and more particularly to a resonant cavity device responsive to applied signals of high frequency.

Wave signal selectors are used in radio apparatus to pass, with favorable amplitude, a selective range of frequencies while greatly attenuating certain other frequencies. When the signals applied are in the range of about 100 megacycles and above, resonant cavity devices often provide more satisfactory tuning than do circuits with lumped constant combinations. Because the physical dimensions of the components of the resonator directly determine its resonant frequency, it is essential to hold close tolerances in the cavity and components therein. This results in relatively high manufacturing cost. To reduce the criticality of tolerances, the resonant cavity is often end loaded by a variable capacitor to permit adjusting the resonant frequency thereof to the precise levels desired. This also permits cavities to be tuned over a desired frequency range.

A major application for resonant cavity devices is for radio frequency (RF) coupling. A cavity resonator can be used, for example, to couple the antenna to the first audio frequency amplifier stage of a radio receiver, or to couple subsequent RF stages of the receiver. It may be desired to use a device having a plurality of resonant cavities with the energy passing through successive ones of the resonant cavities.

Various difficulties in the mechanical construction of a a resonant cavity device may arise. As mentioned above, close tolerances are required, and machining the cavities to close tolerances is expensive. Certain applications of resonant cavity devices may require that physical dimensions be minimized, compounding manufacturing difliculties. Often resonant cavity devices have been expensive to construct and assemble, and where a plurality of resonant cavities are utilized, the coupling between such cavities may result in a high insertion loss.

Accordingly, it is an object of the present invention to provide an improved resonant cavity device which overcomes one or more of the objections to the prior art devices.

A further object of the invention is to provide an improved resonant cavity device which is low in cost and simple to manufacture.

Still another object is to provide a reliable and sturdy resonant cavity device which occupies a minimum of space.

A further object is to provide a resonant cavity device incorporating a plurality of resonant cavities having low intercavity leakage and high Q, for low insertion loss.

A feature of the invention is the provision of a resonant cavity device and a conductor member extending into the cavity with a variable capacitor at the end thereof, which variable capacitor has concentric cylindrical surfaces and provides support for the free end of the conductor member.

Another feature of the invention is the provision of a resonant cavity device with a rod extending from a housing into a cavity therein, which rod has an end loading capacitor at the free end thereof comprised of concentric cylindrical inner and outer members which are relatively adjustable and which are separated by a supporting spacer.

A further feature of the invention is the provision of a cast housing having a plurality of cavities therein and a removable cover plate at an open end of the cavities,

the housing being constructed to leave space between the cover plate and the separating walls of the cavities for intercoupling thereof.

In the drawings:

FIG. 1 is a side elevation, partially in section, of a resonant cavity device constructed in accordance with the invention;

FIG. 2 is a top view of the device of FIG. 1;

FIG. 3 is a sectional view taken along the line 33 of FIG. 1; and

FIG. 4 is an exploded view of the inner conductor memher and end loading capacitor of the device of FIGS. 1, 2 and 3.

In accordance with the invention, a resonant cavity device for use in a circuit responsive to applied signals of high frequency includes a housing forming a conductive cavity, and an elongated conductor member extending into the cavity. A removable plate is provided at one end of the cavity, and one end of the conductor member is fixed to the plate. An adjustable capacitor is provided at the end of the conductor member opposite the plate and includes inner and outer elements, one of which is mounted on the conductor member and the other is mounted on the housing. The inner and outer elements have concentric cylindrical surfaces, and a spacer of nonconductive material maintains the two elements in spaced relation.

In a particular embodiment of the invention, the cavity is elongated and the conductor member is a rod extending therein. The inner element of the capacitor is a cylinder mounted on the end of the rod, and the outer element is a hollow cylinder concentric with the inner element and adjustably mounted in an opening in the housing. The element mounted on the housing is threadably engaged therewith and has a portion external of the housing to permit turning of the element for axial adjustment of the outer element with respect to the inner element, thereby adjusting the capacitor. The housing may include a plurality of adjacent cavities, with the walls between the cavities at the ends adjacent the removable plate being spaced from the plate for intercavity coupling.

Referring now more particularly to the drawings, FIG. 1 shows a resonant cavity device, comprising a plurality of adjacent cavities, constructed in accordance with the invention. The resonant cavity device shown comprises a housing including front wall 11, back wall 12, side walls 13 and 14, and top wall 15. The housing is completed by a bottom plate 1 6 which is removably secured by integral flanges 17 to side walls 13 and 14 of the housing. Plate 16 is attached to the housing at the bottom of walls 11 and 12 by means of self-tapping screws 20. The screw holes are cast into the housing but are not tapped in order to reduce machining cost. The screw holes do not extend completely through the housing walls to prevent metal chips produced by the self-tapping screws from falling inside the cavity housing. The housing includes a plurality of interior walls 18 which divide the interior of the housing into cavities. The ends of interior Walls 18 adjacent plate 16 are spaced from the plate a predetermined distance according to the amount of intercavity coupling desired.

For optimum cost saving it is preferred that that part of the housing defined by walls 11- 15 and 13 be die-cast as an integral unit. Final tolerance requirements may be met by machining if such is necessary. Because intercavity coupling takes place at the end of interior Walls 18 adjacent removable plate 16, there is no critical mating problem involved. This avoids intercavity leakage and insertion loss as contrasted with a construction wherein intercavity coupling is accomplishing elsewhere and a flush mate between the ends of interior walls 18 and 16 is necessary to prevent leakage.

Plate 16 carries a plurality of conductive rods 21 which are mounted at one end thereof to plate 16, such as by staking, welding or soldering. External electrical connection to and from the first and last cavities is made by means of conductors 2 2 fastened to respective ones of rods 21, such as by soldering. A slot may be cut into rods 21 to provide an exact connecting point for conductors 22 prior to soldering. Conductors 22 are connected to the interior contact ring 23 of connector jacks 24. Jacks 24 are carried in the flange portions 17 of plate 16 and mate with U-shaped slots 25 provided in the side walls 13 and 14 of the housing.

Conductor 21, together with the Walls of the particular cavity in which they are disposed, have distributed inductance and capacity which form a resonant combination at a particular desired frequency depending upon their dimensions. Each cavity is end loaded by means of an adjustable capacitor which provides variation of the resonant frequency to precisely the desired level. The adjustable capacitor is comprised of a cylindrical inner member 31 mounted on the free end of rod 21 and a hollow cylindrical outer member 32. The outer member 32 is externally threaded and is disposed through a threaded opening in top wall to be concentric with inner member 31. Accurate spacing between the inner walls of member 32 and the outer walls of member 31 is maintained by means of a pacer 33 of a non-conductive material such as, for example, a polytetra-fiuoroethylene resin. Spacer 33 is mounted to the end of inner member 31 by a rivet 34 or drive pin.

It will be apparent that even minor variations in the position of outer member 32 with respect to inner member 31 will vary the capacity of the resonant cavity. In order to avoid the effect of backlash between the threads of outer member 32 and the threaded opening in top wall '15, means for preventing axial movement due to backlash are provided. A pin 41 is set in front wall 11 of the housing adjacent each of outer members 32. A spring washer 43 is disposed above that portion of each outer member external of the housing and is compressed by a threaded washer 45 in which outer member 32 is threaded. A slot 47 in each of washers 45 mates with the corresponding pin 41 to prevent washer 45 from turning when outer member 32 is turned. Spring washer 43, therefore, provides a spring bias outwardly for outer member 32, holding member 32 constantly at one extremity of the play permitted by the backlash between the threads on outer member 32 and the threads of the opening in top wall 15. This prevents variation in the relative positions of outer member 32 with respect to inner member 31.

The foregoing described construction for a resonant cavity device is such that material and assembly cost is very low while a superior level of performance is maintained. The housing may be cast with a minimum of machining necessary, and the interior elements of the cavity such as rods 21 and inner and outer members 31 and 32 respectively are susceptible of automated manufacture from standard rod stock with a minimum of waste. Rods 31 are securely supported at either end since the spacer 33 engages the inner walls of outer member 32. Thus capacity changes due to shock and vibration are minimized. In addition, the rods 21 may be staked, welded or soldered to plate 16, alleviating the necessity for extra mounting hardware such as screws, rivets etc.

From the foregoing discussion it may be seen that the resonant cavity tuner of the invention is low in cost and simple of manufacture. Furthermore, the unit is sturdy and reliable while occupying a minimum of space. Low intercavity leakage results, thus minimizing insertion loss. Units in accordance with the invention may be manufactured with a single cavity or with any number of coupled cavities desired. Accurate adjustment of the capacity is readily obtained through exterior means.

I claim:

1. A resonant cavity device responsive to applied signals of high frequency, including in combination, a hollow die-cast conducting housing having a plurality of walls forming a plurality of cavities therein, a removable end plate at one end of aid housing, said walls of said housing between adjacent cavities being formed to leave a space between said walls and said end plate for intercavity coupling, a plurality of conductive members each extending into a respective one of said cavities and fixed at one end thereof to said end plate, and an adjustable capacitor at the free end of each of said conductive members, said adjustable capacitor comprising a first portion mounted on the free end of said conductive member and a second portion supported in said housing, said first and second portions having concentric cylindrical surfaces, each of said capacitors further comprising non-conductive separator means on one of said first and second portions thereof engaging the other of said portions to maintain said concentric cylindrical surfaces in spaced relation, said second portions each having a part external of said housing for adjusting said portions axially with respect to said first portions thereby adjusting said capacitors.

2. The resonant cavity device of claim 1 further including a threaded washer external of said housing and threadably engaged with said part of said second portion external of said housing, said threaded washer being fixed rotationally with respect to said housing, and a spring washer biasing said threaded washer outwardly of said housing to prevent axial movement of said outer member with respect to said inner member due to thread backlash.

3. A resonant cavity device responsive to applied signals of predetermined frequencies, including in combination, a hollow die-cast integral conducting housing having a plurality of walls forming a plurality of cavities therein and being open at one end, a removable end plate for covering said one end of said housing, said walls of said housing between adjacent cavities being formed to leave a space between said walls and said end plate for intercavity coupling, a plurality of conductive members each extending into a respective one of said cavities and fixed at one end thereof to said end plate, and adjustable capacitor means supported in said housing and being adjusted to vary the resonant frequency of each of said cavities, and each of said conductive members being securely supported at one end by said end plate and at the other end by said capacitor means thereby minimizing variations in the resonant frequency of said cavities caused by vibration of said housing.

References Cited by the Examiner UNITED STATES PATENTS 2,417,542 3/1947 Carter 33383 2,500,875 3/1950 Schupbach 33382 2,503,256 4/1950 Ginzton 333-83 2,833,994 5/1958 Pipes 33382 OTHER REFERENCES Electronic Industries, vol. 20, August 1961, pages 108-109.

Electronic Products Magazine, October 1961, page 21. Microwave Journal, vol. 5, No. 10, October 1962, page. 21.

HERMAN KARL SAALBACH, Primary Examiner.

ELI LIEBERMAN, Examiner.

L. ALLAHUT, Assistant Examiner. 

3. A RESONANT CAVITY DEVICE RESPONSIVE TO APPLIED SIGNALS OF PREDETERMINED FREQUENCIES, INCLUDING IN COMBINATION, A HOLLOW DIE-CAST INTEGRAL CONDUCTING HOUSING HAVING A PLURALITY OF WALLS FORMING A PLURALITY OF CAVITIES THEREIN AND BEING OPEN AT ONE END, A REMOVABLE END PLATE FOR COVERING SAID ONE END OF SAID HOUSING, SAID WALLS OF SAID HOUSING BETWEEN ADJACENT CAVITIES BEING FORMED TO LEAVE A SPACE BETWEEN SAID WALLS AND SAID END PLATE FOR INTERCAVITY COUPLING, A PLURALITY OF CONDUCTIVE MEMBERS EACH EXTENDING INTO A RESPECTIVE ONE OF SAID CAVITIES AND FIXED AT ONE END THEREOF TO SAID END PLATE, AND ADJUSTABLE CAPACITOR MEANS SUPPORTED IN SAID HOUSING AND BEING ADJUSTED TO VARY THE RESONANT FREQUENCY OF EACH OF SAID CAVITIES, AND EACH OF SAID CONDUCTIVE MEMBERS BEING SECURELY SUPPORTED AT ONE END BY SAID END PLATE AND AT THE OTHER END BY SAID CAPACITOR MEANS THEREBY MINIMIZING VARIATIONS IN THE RESONANT FREQUENCY OF SAID CAVITIES CAUSED BY VIBRATION OF SAID HOUSING. 